This invention relates to a jointed body of glass-ceramic and aluminum nitride sintered body and method for producing the jointed body.
Recently, use of microwave of 1 GHz, such as microwave band and quasimillimetric wave band, is increasing in the field of wireless communication, e.g., portable telephones, and optical communication.
As semiconductors operated in such a high-output, high-power consumption Gaxe2x80x94As FET, Sixe2x80x94Ge HBT, CMOS or GaN laser diode and the like are started to be used. Electric circuits on which such semiconductors are to be mounted must meet such requirements as 1) that the circuit pattern material shows small electric resistance, 2) that multi-layered circuit can be formed for size reduction, 3) that the substrate insulation material has a high heat conductivity, high electric insulation and desirably low dielectric constant, and 4) that environmental pollution by circuit substrate material is little.
However, there is no single insulating substrate material meeting all of these conditions. Multi-layered circuit substrate using a highly thermoconductive aluminium nitride sintered body is available, but its electric circuit conductor is made of tungsten/molybdenum type material having high electric resistance and is unsuitable for high-frequency circuits.
A conceivable realistic solution of above problem is a composite circuit substrate in which aluminum nitride sintered body is joined with glass. That is, aluminum nitride sintered body functions to spread the heat from semi-conductors, and further in which a part of an electric circuit is formed; a mono-layered or multi-layered glass layer is formed thereon using paste-printing technique or the like: and an electric circuit is formed with Au-, Ag-, or Cu-derived low resistance material on the surface or inside of said glass layer.
A glass material for realizing such a circuit substrate must possess such properties as: 1) high electrical insulation and favorable dielectric characteristics, 2) thermal expansion coefficient close to that of aluminum nitride sintered body, 3) direct jointability with aluminum nitride sintered body, not mediated by an oxide film, 4) inclusion of crystallized phase to enable repeated sintering, and 5) relatively low jointing temperature with aluminum nitride sintered body, which must not be higher than the melting point of the metal component of Au-, Ag- or Cu-metallized material, specifically, not higher than 1100xc2x0 C., in other words, the operation temperature for softening the glass to achieve sufficiently intimate jointing with aluminum nitride sintered body must not be higher than 1100xc2x0 C.
As low temperature-softening glass, those containing lead oxide can be produced with relative ease, but lead-containing glass is fundamentally toxic and significantly adversely affects terrestrial environments, although lowering in softening point can be easily achieved therewith. Furthermore, lead-containing glass is apt to have a high thermal expansion coefficient. It also has a defect of readily inducing foaming phenomenon under high temperatures at which it reacts with aluminum nitride sintered body and the reaction gas remains in the glass.
As glass materials for the circuit substrates, those which are described in JP-A-340443/1994 and JP-B-68065/1995 are known. The glass in JP-A-340443/1994 contains a large amount of titanium or zirconium and lead, and the defect of higher thermal expansion coefficient than that of aluminum nitride sintered body, which is 4.5xc3x9710xe2x88x926/xc2x0 C., is not fully removed. The glass also exhibits low joining strength with aluminum nitride substrate. The glass in JP-B-68065/1005 exhibits favorable thermal expansion coefficient and electric insulation, but requires a high temperature treatment at 1100-1500xc2x0 C. for obtaining satisfactory jointability with aluminum nitride. Therefore, two or more steps are necessary for incorporating an electric circuit therein.
The present invention is made for solving such problems as above-described. I noticed an anorthite (CaO.Al2O3.2SiO2) sintered body has, as taught in an article by R. A. Gdula, American Ceramic Society Bulletin, 1971, Vol.50, No.6, 555-557, a high electric insulation (2.8xc3x971015 xcexa9.cm: 25xc2x0 C.), a small dielectric constant (6.2: 1 MHz, 25xc2x0 C.), a thermal expansion coefficient (48xc3x9710xe2x88x927/xc2x0 C.: 150xc2x0 C.-700xc2x0 C.) which is close to that of aluminum nitride sintered body, and furthermore stability in reducing atmosphere such as of H2, and conducted concentrative studies thereof.
In consequence, I discovered amorphous glass which contains Ca, Si and Al components and hence is expected to be crystallized into anorthite crystals under heating has low melting point in the absence of a large amount of Pb component, exhibits good jointability with aluminum nitride sintered body at relatively low temperatures not higher than 1100xc2x0 C. and crystallizes under heating to separate a crystal component having a characteristic strongest line in a range of 2"THgr"=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line. Moreover, I also found that the crystallized glass or glass-ceramic containing said crystal component possessed not only those characteristic properties attributable to said anorthite sintered body but also high thermal stability well withstanding repeated heating for forming a multi-layered glass. (In the following, an amorphous glass having the composition as will form the crystals showing the strongest line in a range of 2"THgr"=27.6xc2x0 to 28.2xc2x0 under heating may occasionally be referred to as xe2x80x9cstarting glassxe2x80x9d.
I furthermore found that use of Zn component-containing glass as the starting glass could further lower the crystallization temperature of the crystals having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line, by 25-100xc2x0 C. or even more, and could also sharpen the separation pattern of said crystals. It was whereby made possible to prepare a jointed body of glass-ceramic with aluminum nitride sintered body at still lower temperatures, with less occurrence of swelling or residual. carbon in so produced glass-ceramic.
Moreover, I also found that a glass of above-described composition could be sufficiently crystallized, without containing a large amount of Ti or Zr component as nucleating agent and hence the glass-ceramic prepared therefrom could have a thermal expansion coefficient nearly the same to that of aluminum nitride sintered body.
Namely, the present invention provides a jointed body of glass-ceramic consisting of crystalline portion and amorphous portion with aluminum nitride sintered body, which is characterized in that the crystalline portion contains as main crystals a crystal having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line and said glass-ceramic contains 0.5-30% by weight of a Zn component in terms of oxide, not more than 10% by weight of the sum of a Ti component and Zr component in terms of respective oxides and not more than 5% by weight of a Pb component in terms of oxide.
The invention also provides a method for producing a jointed body of glass-ceramic with aluminum nitride sintered body, which method is characterized by forming a glass layer containing an amorphous glass having a composition of 0.5-30% by weight of a Zn component in terms of oxide, not more than 10% by weight of the sum of a Ti component and Zr component in terms of respective oxides and not more than 5% by weight of a Pb component in terms of oxide, on an aluminum nitride sintered body, heating the resulting composite to a temperature not lower than the softening point of said amorphous glass and whereby crystallizing said glass to convert it to a glass-ceramic, the main crystals therein having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line. The present invention also covers an embodiment of the method for producing the jointed body in which, in addition to the glass-ceramic and aluminum nitride sintered body, an electric circuit layer is jointed by heating.
The invention furthermore provides a glass-ceramic composed of amorphous portion and crystalline portion, which can be favorably used in said jointed bodies and the like, said crystalline portion containing as main crystals a crystal having the strongest line in the range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line, and said glass-ceramic being composed of 8-25% by weight of CaO, 15-35% by weight of Al2O3, 33-55% by weight of SiO2, 0.05-18% by weight of B2O3 and 0.5-25% by weight of ZnO.
The glass-ceramic constituting the jointed body of the present invention consists of crystalline portion and amorphous portion.
Compounds consisting solely of crystalline portion have excessively high softening points, and are very difficult to be jointed with aluminum nitride sintered body. Glass-ceramic containing amorphous portion can be jointed with aluminum nitride sintered body at relatively low temperatures, but it is extremely difficult to completely eliminate the amorphous portion by 100% crystallization. On the other hand, glass consisting solely of amorphous portion has a high thermal expansion coefficient and is inferior in electric characteristics and chemical resistance.
Said crystalline portion contains a crystal having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line (which crystal may hereafter be referred to as xe2x80x9cA crystalxe2x80x9d) forming the main crystal phase. A typical example of such a crystal component is, as illustrated by FIG. 1, anorthite crystal (CaO.Al2O3, 2SiO2; CaAl2Si2O8) which is identified as Powder Diffraction File No. 20-20 of JCPDS (Joint Committee On Powder Diffraction Standards).
When main crystals separated are those (A crystals) having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line, the following effects can be achieved. That is, anorthite crystal which is a typical A crystal as aforesaid has high electric insulation and small dielectric constant, and hence exhibits excellent electric characteristics as a substrate for electric circuits. Also because its thermal expansion coefficient is close to that of aluminum nitride, that of the glass-ceramic as a whole can also be made of same level with aluminum nitride sintered body with ease. Anorthite crystal is stable under heating in a reducing atmosphere, which renders its repeated heating possible, for forming a multi-stratified glass layer. Moreover, chemical resistance of the glass layer also is drastically improved compared to amorphous glass.
When a Zn component is blended in a starting glass, depending on the heating conditions crystals having the strongest line in a range of 2xcex8=36.6xc2x0 to 37.0xc2x0 in powder X-ray diffraction using CuKxcex1 line (which crystal may be hereafter referred to as xe2x80x9cB crystalxe2x80x9d) may be separated, besides A crystals, as the crystalline portion of the glass. A typical example of B crystal is gahnite crystal (ZnO, Al2O3, 2SiO2; ZnAl2O4) which is identified as Powder Diffraction File No. 5-669 of said JCPDS.
In crystalline portion of the glass-ceramic following the present invention thus A crystals having the strongest line in a range of 2xcex8=27.6xc2x0 to 28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line are separated, and B crystals having the strongest line in a range of 2xcex8=36.6xc2x0 to 37.0xc2x0 using powder X-ray diffraction using CuKxcex1 line may also be separated. Whereas, according to the present invention A crystals must be the main crystals, because while anorthite crystal which is the representative of A crystal has a thermal expansion coefficient of 4.8xc3x9710xe2x88x926/xc2x0 C., a value close to that of aluminum nitride sintered body, gahnite crystal which is the representative of B crystal has a high thermal expansion coefficient of 7.7xc3x9710xe2x88x925/xc2x0 C., and where B crystal content is high, it becomes very difficult to make thermal expansion coefficient of the glass-ceramic equivalent to that of aluminum nitride sintered body. Here xe2x80x9cmain crystalsxe2x80x9d signifies that the amount of A crystals as determined following the later specified method for measuring crystallization amount is at least 50% by weight, preferably at least 60% by weight, inter alia, at least 70% by weight of the crystalline portion.
Separation of B crystals, on the other hand, is not essential, but preferably B crystals occupy 10-40% by weight, preferably about 10-30% by weight, of the crystalline portion. There is a tendency that the more B crystals, the more A crystals. However, excessively high B crystal content is objectionable because it has a higher thermal expansion coefficient than A crystal.
It is sufficient for the glass-ceramic following the present invention that the total amount of crystals is of the level allowing confirmation by powder X-ray diffraction using CuKxcex1 line. Whereas, from the viewpoint of thermal expansion coefficient, electrical characteristics, durability and softening point, total amount of A crystals and B crystals as determined by the later specified measurement method of crystallization is preferably 1-90% by weight; in particular, 3-85% by weight, inter alia, 30-80% by weight. The best result can be obtained when it is 40-70% by weight. Glass of high crystal content tends to require high temperatures for its production or fail to give sufficient jointing strength with aluminum nitride sintered body. On the other hand, there is seen a tendency that the less the crystal content, the higher the thermal expansion coefficient and the lower the electrical characteristics, chemical resistance, durability, etc.
It is preferred that the total crystalline portion is 30-80% by weight, of which 60-90% by weight being A crystals and 10-40% by weight being B crystals. Even better is that the total crystalline portion is 40-70% by weight, of which 70-90% by weight being A crystals and 10-30% by weight being B crystals.
So long as the effects of the present invention are not impaired, crystal(s) other than one having the strongest line in the ranges of 2xcex8=27.6xc2x0 to 28.2xc2x0 and 2xcex8=36.6xc2x0 to 37.0xc2x0 in powder X-ray diffraction using CuKxcex1 line, may be separated.
The respective amounts of A crystals and B crystals in a glass-ceramic following the present invention can be determined by the following method, which is explained referring to anorthite, a typical example of A crystal, and gahnite, a typical example of B crystal.
First, the end point of crystallization is ascertained by means of powder X-ray diffraction and differential thermal analysis. Where an amorphous glass is heated to be crystallized, until the condition is reached at which the crystallization no more progresses even under higher temperatures or longer hours"" heating. That condition is specified as the one at which the maximum anorthite and gahnite contents are attained. The intensity ratio between the strongest lines of anorthite and gahnite in powder X-ray diffraction (when CuKxcex1 line is used, the strongest line of anorthite appears in the range of 27.6xc2x0 to 28.2xc2x0, and that of gahnite, in the range of 36.6xc2x0 to 37.0xc2x0) is recorded as the molar ratio of anorthite crystal to gahnite crystal (the molar ratio is hereafter called R). Then, of the components contained in the glass-ceramic of the present invention, Al2O3 which is a constituent of both anorthite crystal and gahnite crystal is distributed to said two according to R. Where no gahnite crystal is present, it is unnecessary to distribute the Al2O3, but all is calculated as anorthite crystal component.
Then the maximum amount of crystallization in the glass-ceramic is calculated. For calculating the maximum amount of anorthite, the least component among those constituting anorthite, i.e., CaO, Al2O3 and SiO2, in terms of molar ratio is taken as the standard, under a presumption that all of the least content component is consumed for crystallization of anorthite and the remaining two components are consumed at a molar ratio of CaO: Al2O3:SiO2=1:1:2 to form anorthite crystals, and the total amount of CaO, Al2O3 and SiO2 at this time is the maximum amount of separated anorthite crystals as referred to in the present invention. According to the present invention, furthermore, the Al component is calculated by a unit of Al2O3, and the Al2O3 content used in the above calculation is same as the calculated amount as distributed to anorthite according to R as above-specified.
On the other hand, calculation of the maximum amount of gahnite crystals is based on the less component between those constituting gahnite, i.e., ZnO and Al2O3, in terms of molar ratio, under a presumption that all of the least content component is consumed for crystallization of gahnite and that the other component is consumed at a molar ratio of ZnO:Al2O3=1:1 to form gahnite crystals. The total amount of ZnO and Al2O3 at this time is the maximum amount of gahnite crystals as referred to in the present invention. The Al2O3 content used in this calculation is the same to that which is distributed to gahnite crystals according to R as above-specified.
For example, taking a case of a glass containing CaO: 15 wt % (2.68 mmol/g), Al2O3: 24 wt % (2.35 mmol/g), SiO2: 42 wt % (7.00 mmol/g), B2O3: 8 wt % (1.14 mmol/g) and ZnO: 11 wt % (1.36 mmol/g), its crystallization from amorphous condition is deemed to be completed by heating at 900xc2x0 C. The ratio (R) of anorthite to gahnite in the glass-ceramic after said heating at 900xc2x0 C. is 0.3769.
Then the Al2O3 content of 24 wt % (2.35 mmol/g) is distributed according to R=0.3769, i.e., 17.43 wt % (1.71 mmol/g) to anorthite crystals and 6.57 wt % (0.64 mmol/g) to gahnite crystals.
The glass composition capable of separating anorthite crystals is CaO: 15 wt % (2.68 mmol/g), Al2O3: 17.43 wt % (1.71 mmol/g) and SiO2: 42 wt % (7.00 mmol/g), and therefore the maximum amount of anorthite crystals is calculated following the content of Al2O3, the least component, to be the sum of CaO: 9.58 wt %, Al2O3: 17.43 wt % and SiO2: 20.54 wt %, which equals 47.55 wt %.
Because ZnO: 11 wt % (1.36 mmol/g) and Al2O3: 6.57 wt % (0.64 mmol/g), the sum of ZnO: 5.24 wt % (0.64 mmol/g) and Al2O3: 6.57 w % (0.64 mmol/g) which equals 11.81 wt % is the maximum amount of gahnite crystals.
Hence, the maximum crystal content in this glass-ceramic, anorthite crystals and gahnite crystals, as combined is 59.36 wt %.
The remaining CaO: 5.42 wt %, SiO2: 21.46 wt %, ZnO: 5.76 wt % and B2O3: 8 wt % are calculated to form amorphous glass matrix.
A crystal content in a glass-ceramic can be calculated from the maximum crystal content as calculated by the above method, X-ray diffraction intensity and measured X-ray diffraction intensity.
For example, starting from a starting glass composed of above components and which is in amorphous condition, its heating temperature is continuously raised until its anorthite and gahnite crystal contents reached the maximum. At that time the powder X-ray diffraction intensity (count number) indicated by the strongest line inherent in anorthite crystal is 1170, and that (count number) indicated by the strongest line inherent in gahnite crystal is 441.
Where said X-ray diffraction intensity (count number) of the starting glass under heating, in which anorthite crystals alone are separated, is 55, the anorthite crystal content of the glass under heating is calculated to be 2.24 wt % (47.55xc3x9755/1170=2.24).
When gahnite crystals also start to be separated under further heating and X-ray diffraction intensity (count number) of the gahnite crystals is 90, the gahnite crystal content in the glass-ceramic under said condition is calculated to be 2.41 wt % (11.81xc3x9790/441). If the X-ray diffraction intensity of anorthite crystals in the glass containing 2.41 wt % of gahnite crystals and being heated is 1116, the anorthite crystal content of the glass-ceramic is calculated to be 45.36 wt %, and the crystal content, which is the sum of said anorthite crystals and gahnite crystals, of the glass-ceramic is 47.77 wt %.
In the aforesaid glass under heating in which anorthite crystals alone are separated in an amount of 2.24 wt %, its crystalline portion consists of CaO: 0.45 wt %, Al2O3: 0.82 wt % and SiO2: 0.97 wt %. The composition of the amorphous portion is: CaO: 14.55 wt %, Al2O3: 23.18 wt %, SiO2: 41.04 wt %, B2O3: 8 wt % and ZnO: 11 wt %.
Also in the glass wherein 45.36 wt % of anorthite crystals and 2.41 wt % of gahnite crystals are separated, the anorthite crystal portion consists of CaO: 9.14 wt %, Al2O3: 16.62 wt % and SiO2: 19.59 wt %; and the gahnite crystal portion consists of Al2O3: 1.34 wt % and ZnO: 1.07 wt %. The amorphous portion contains CaO: 5.86 wt %, Al2O3: 6.04 wt %, SiO2: 22.41 wt %, B2O3: 8 wt % and ZnO: 9.93 wt %.
Where A crystal and B crystal are other than anorthite and gahnite, respectively, their compositions can be determined in a manner similar to the above-described method.
Glass-ceramic containing anorthite crystals, which is to be used in the present invention, normally contains Ca, Al and Si. These components are considered to be crystallized as anorthite crystals to show the strongest line appearing in powder X-ray diffraction using CuKxcex1 line in the range of 2xcex8=27.6xc2x0-28.2xc2x0. These components may also be present in the amorphous portion.
Ca component is preferably contained in glass-ceramic in an amount as converted to CaO of 5-25 wt %, in particular, 8-25 wt %. Where the Ca content is too low, it becomes difficult to separate crystals which are inferred to be anorthite crystals in glass-ceramic. On the other hand, when the amount is excessive, jointability of the glass-ceramic with aluminum nitride sintered body is impaired, e.g., cracks are formed during the jointing operation.
Preferred Al component content in terms of Al2O3 is 15-40 wt %, in particular, 15-35 wt %. Where the Al component content is too low, anorthite crystals are difficult to be separated, but when it is too much, there observed a tendency that high temperatures are required for jointing with an aluminum nitride sintered body.
Too low a Si component content also renders it difficult to separate anorthite crystals. Conversely, when it is too high, higher temperatures tend to be required for jointing the glass-ceramic with aluminum nitride sintered body. Si component content is preferably 25-60 wt % in terms of SiO2, in particular, 33-55 wt %.
It is important that glass-ceramic according to the present invention contains Zn component. By incorporating a Zn component, it becomes possible to lower the crystallization temperature of A crystals which have the strongest line in powder X-ray diffraction using CuKxcex1 line in the range of 27.6xc2x0-28.2xc2x0, by 25-100xc2x0 C. compared to the case where no Zn component is incorporated. Furthermore, the incorporation sharpens crystallization pattern of said crystals, and hence can prevent occurrence of residual carbon or swelling in formed glass-ceramic.
In occasions said Zn component may separate as, for example, gahnite crystals. In consequence, excessive Zn component causes crystallization of greatest part of Al component as B crystals, e.g., gahnite crystals, although consumption of the Al component for A crystals is preferred. Therefore, Zn component content in terms of ZnO is preferably 0.5-30 wt %, in particular, 0.5-25 wt %. ZnO can also be present in the amorphous portion.
Glass-ceramic according to the present invention preferably contains a boron (B) component as incorporated therein, to have a lowered softening point and improved jointability with aluminum nitride sintered body which is described later. Whereas, when a large amount of boron (B) component is contained, detrimental effects may be brought, such as increase in hygroscopicity and reduction in chemical resistance, and preferably the content in terms of B2O3 is in a range of 0.05-20 wt %, in particular, 0.05-18 wt %. B2O3 is present in amorphous portion of glass-ceramic.
It is a general practice to blend a Ti and/or Zr component as a crystallization or nucreating agent, for crystallization of crystallizable component(s) in glass-ceramic. Ti and/or Zr component, however, increases thermal expansion coefficient of glass-ceramic, and its or their blending in a large amount is undesirable. Furthermore, when these components are contained in large amounts, they may cause separation of CaO.TiO2.SiO2 crystals, CaO.ZrO2.2TiO2.SiO2 cryastals or ZrO2.SiO2 crystals to interfere with separation of the crystals having an inherent strongest line in powder X-ray diffraction using CuKxcex1 line in the range of 2xcex8=27.6xc2x0-28.2xc2x0. According to my studies, by inclusion of above-listed CaO, Al2O3, SiO2, ZnO and B2O3 as the constituent components, it becomes possible to separate a sufficient amount of crystals in the glass-ceramic, in the complete absence of Ti and/or Zr component. For approximating thermal expansion coefficients of glass-ceramic and aluminum nitride sintered body, it is necessary to keep blended amount of Ti and/or Zr component not higher than 10 wt % in terms of oxide(s), in particular, not higher than 5 wt % in total, inter alia, substantially zero.
According to the invention, the glass-ceramic is jointed with aluminum nitride sintered body. Generally a Pb component is blended in the glass to lower softening point of glass and facilitate its jointing with ceramics. Pb, however, reacts with aluminum nitride at high temperatures to evolve a gas, whereby causing swelling or the like in glass-ceramic. Therefore, less Pb component contained in the glass is preferred. More specifically, it must be no more than 5 wt % in terms of oxide thereof. It is preferred that substantially no Pb component is present in the glass.
It is permissible for the glass-ceramic according to the present invention to contain, besides those components so far described, another component or other components for the purpose of lowering softening point of the glass or improving its jointability or adherability to aluminum nitride sintered body. As examples of such additional components, alkalu metals (Li, Na, K) or alkaline earth metals (Mg, Sr, Ba) may be named. When these components are blended in large amounts, however, deterioration in electrical properties (for example, dielectric constant or electric resistance), reduction in chemical resistance or increase in thermal expansion coefficient result.
Again, so long as the effects of the present invention are not impaired, other oxide component(s) may be blended.
It is preferred that the content of such component(s) other than CaO, Al2O3, SiO2, ZnO, B2O3, PbO, TiO2, ZrO2 is not more than 7 wt % in terms of corresponding oxides. In particular, alkali metal component is preferably not more than 2 wt %, and alkaline earth metal component, not more than 5 wt %, both in terms of their respective oxides. It is more convenient that the sum of an alkali metal component and alkaline earth metal component be not more than 5 wt %, even still better is that the glass is substantially free of these components. Absence of these components creates no problem.
Glass-ceramic according to the invention may further contain a fluorine component which cannot be converted to an oxide, within a range not interfering with the effects of the invention.
Considering the balance among such factors as ready availability of starting materials and ease of production which are discussed later; electric characteristics such as electrical resistance, dielectric constant; jointability with aluminum nitride sintered body which also is observed later; ready coincidence in thermal expansion coefficients between the glass-ceramic and said sintered body; thermal resistance and chemical resistance; preferred composition of the glass-ceramic according to the present invention is as follows, in which all weight percents are in terms of respective oxides: CaO: 5-25 wt %, Al2O3: 15-40 wt %, SiO2: 25-60 wt %, B2O3: 0.05-20 wt %, ZnO: 0.5-30 wt %, TiO2+ZrO2: 0-5 wt %, PbO: 0-5 wt %, and other metal oxide(s): 0-7 wt %, (the total of these components being 100 wt %); in particular, CaO: 8-25 wt %, Al2O3: 15-35 wt %, SiO2: 33-55 wt %, B2O3: 0.05-18 wt % and ZnO: 0.5-25 wt % (the total of these components being 100 wt %). Furthermore, in the optimum composition, ZnO is 3-18 wt %.
The glass-ceramic of the present invention, which contains the crystals having the strongest line in powder X-ray diffraction using CuKxcex1 line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 has electrical characteristics of high electrical resistance and small dielectric constant, i.e., normally an electric resistance of at least 1xc3x971012 xcexa9.cm and a dielectric constant not higher than 8 at 1 GHz. As stated later, to allow an electric circuit which is formed on glass-ceramic or aluminum nitride sintered body fully exhibits its characteristics, the glass-ceramic preferably has an electrical resistance of at least 1xc3x971013 xcexa9.cm, in particular, at least 1xc3x971014 xcexa9.cm. Also its dielectric constant is preferably not more than 7.5 at 1 GHz, in particular, not more than 7. Particularly suitable glass-ceramic for the jointed body of the present invention has an electrical resistance of at least 1xc3x971014 xcexa9.cm and a dielectric constant of not more than 7 at 1 Ghz.
Glass-ceramic of the present invention can be prepared by a method known per se. More specifically, it can be prepared by preparing an amorphous glass of a composition capable of being crystallized to separate the crystals having the strongest line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line under heating and heating the same to a prescribed temperature to crystallize the glass.
The amorphous glass has a softening point and crystallization temperature preferably not higher than 1100xc2x0 C., in particular, not higher than 1050xc2x0 C., inter alia, not higher than 970xc2x0 C. It is still the best that said point and temperature are not higher than 900xc2x0 C.
Such an amorphous glass can be prepared by once fusing the starting materials which are formulated to give an above-described chemical composition, and thereafter rendering it amorphous by such means as quenching to provide a glass.
As the starting materials to be used in the above production method of said amorphous glass, oxides corresponding to the intended composition of the glass can be named. Those oxides are not limited to simple oxides but complex oxides may also be used. Again as the starting materials useful in the above production method, carbonates and hydroxides can also be suitably used besides oxides. Furthermore, halides such as fluorides, chlorides and the like; inorganic salts such as nitrates and sulfates; organic acid salts such as oxalates and citrates; organometal compounds such as metal alkoxides; and hydrates of the foregoing can also be used.
Specific examples of those compounds other than oxides include CaCO3, Ca(OH)2, calcium acetate, CaCl2, CaF2, Al(OH)3, AlCl3, AlF3, magnesium carbonate, magnesium acetate, MgCl2, MgF2, H3BO3, tetraethyl silicate and the like. As SiO2, natural product such as silica sand may also be used.
Describing the above production method in further details, those starting compounds are weighed to provide an intended composition in terms of oxides and mixed, and fused at around 1200-1700xc2x0 C. The fused composition is quenched and vitrified by such means as flowing over a water-cooled metal plate. This glass can be confirmed to be amorphous by X-ray diffration analysis. The X-ray diffraction chart of the glass is broad in the range of 2xcex8=18xc2x0-35xc2x0 when CuKxcex1 line is used, showing a pattern having a broad peak in the vicinity of 25xc2x0 (FIG. 2).
This amorphous glass is crystallized under suitable heating conditions to separate crystals having the strongest line in a range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line (A crystals) (FIGS. 4 and 5). Depending on selected composition and heating conditions, in addition to A crystals, crystals having the strongest line in a range of 2xcex8=36.6xc2x0-37.0xc2x0 in powder X-ray diffraction using CuKxcex1 line (B crystals) are separated (FIGS. 6-9).
A jointed body according to the present invention is formed by jointing above-described glass-ceramic with aluminum nitride sintered body.
As aluminum nitride sintered body which is used for the jointed body of the present invention, any known sintered bodies can be used without any limitation, so long as their main component has a composition of AlN. As components other than AlN, for example, Y2O3, Yb2O3, Er2O3, Ho2O3 and compounds of rare earth elements containing Sc, Y, Er, Yb, Dy, Ho, Gd, La and the like; and alkaline earth metal compounds such as CaO and SrO, can be used as sintering promotor; an alkali metal like Li2O or silicon compound such as SiO2, Si3N4 or SiC can be used for lowering sintering temperature; and further metals such as Mo, W, V, Nb, Ta, Ti and the like or compounds of those metals or carbon-containing compounds may be used for blackening the formed sintered bodies.
Aluminum nitride sintered body may have an as-sintered surface or the surface may be given various treatments such as washing, honing, grinding or mirror-finishing, among which one meeting the purpose of use can be selected for individual occasion.
Taking into consideration such factors as heat conductivity, chemical resistance, thermal expansion coefficient, electrical characteristics, optical characteristics and ease of production of sintered body, use of AlN sintered body substantially free of other components, or AlN sintered body containing 0.1-15 wt % of Y or Er compound in terms of the respective oxides (Y2O3, Er2O3) is preferred. In particular, use of AlN sintered body containing 0.5-10 wt % in terms of corresponding oxide of Y or Er compound is preferred.
Production method of a jointed body of the present invention in which glass-ceramic is joined with aluminum nitride sintered body may be any, but the following method is particularly preferred.
That is, the method comprises forming a glass layer containing an amorphous glass (the starting glass) having a composition capable of being crystallized under heating to separate the earlier described crystals having the strongest line in a range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line, on an aluminum nitride sintered body, and heating the resultant composite to a temperature not lower than the softening point of the starting glass, whereby jointing the glass with the aluminum nitride sintered body. While it is possible to crystallize to separate the crystals having the strongest line in a range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line by a heating separately from the heating to effect the jointing, said simultaneous crystallization under the heating for the jointing is more advantageous for process design and is preferred. Namely, in the occasion of heating an aluminum nitride sintered body on which the starting glass layer is formed, by that single heating the jointed body of glass-ceramic and aluminum nitride sintered body can be produced by using a heating temperature not lower than the softening point of the starting glass and also not lower than the crystallization temperature of the glass to separate crystals having the strongest line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line.
Such a heating temperature differs depending on individual starting glass composition, but 600-1100xc2x0 C., in particular, 800-1050xc2x0 C., are preferred. It is still more convenient that a temperature within a range of 800-970xc2x0 C. is used, inter alia, 800-900xc2x0 C. Where the heating temperature is too low, the jointing or the crystallization tend to become insufficient. Conversely, heating at too high a temperature tends to increase the crystal amount but at the same time, is disadvantageous in energy consumption. Furthermore, in the occasion of forming an electric circuit layer as later described, such metals as Au, Ag, Cu and the like which constitute said electric circuit layer are fused under such high temperature, and formation of adequate circuit pattern becomes difficult.
Where the glass layer is formed of a multi-layered construction or a separate heating is conducted in the later described occasion of electric circuit formation, i.e., when heating is conducted plural times, crystallization does not necessarily occure at the first heating only for jointing an aluminum nitride sintered body with the glass, but it can progress during the subsequent heating(s).
It should be understood that the method of producing a jointed body of the present invention is not limited to the above, but if necessary such a method comprising using as the starting glass a partially crystallized glass and further advancing the crystallization under the heating for softening and joining; or a method comprising repeating glass paste application, heating and softening cycle plural times and conducting a last heating for crystallization, may also be used.
Extent of crystallization can be confirmed by means of powder X-ray diffraction using CuKxcex1 line as aforesaid. According to the invention, crystals having the strongest line in a range of 2xcex8=27.6xc2x0-28.2xc2x0 are separated. Also another kind of crystals having the strongest line in a range of 2xcex8=36.6xc2x0-37.0xc2x0 are also separated, said crystal having the strongest line characteristic thereof at 2xcex8=36.6xc2x0-37.0xc2x0 also shows a second highest intensity line in a range of 2xcex8=31.0xc2x0-31.4xc2x0, as the crystallization progresses.
For forming said starting glass layer on an aluminum nitride sintered body, any method known per se can be used.
Specifically, a method comprising pulverizing the starting glass, mixing it with an organic binder and solvent to form a paste, applying the paste onto an aluminum nitride sintered body surface by such means as screen printing, and heating the same to volatilize the organic components is preferred.
In the occasion of forming the starting glass into a paste, average particle size of the glass powder is preferably 0.1-20 xcexcm, in particular, 0.3-10 xcexcm, inter alia, 0.5-6 xcexcm, from the viewpoint of easy paste production and favorable leveling property of the glass surface formed after the sintering for joining with aluminum nitride sintered body.
For the purpose of approximating thermal expansion coefficients of glass-ceramic and aluminum nitride sintered body and of lowering dielectric constant or improving binder removing efficiency of the glass, it is effective to add a ceramic powder such as of Al2O3, SiO2, Si3N4, SiC, AlN, mullite, spinel, cordierite and the like to the starting glass paste as a filler. It is also permissible to add pigment which contains a transition metal such as iron, cobalt, nickel or chromium, in an amount of a range not detrimental to the characteristic properties of the glass, for example, not more than 5 wt % of a transition metal as converted to its oxide, to color the glass green, blue or brown. Generally spinel type pigments are preferred.
The joining or bonding strength between glass-ceramic and aluminum nitride sintered body in a jointed body of the present invention preferably is at least 25 Mpa, in particular, at least 35 Mpa, inter alia, at least 50 Mpa, as measured by 90xc2x0 C. perpendicular tensile test, from the standpoint of reliability and durability when the body is used as a part of various devices.
In a jointed body of the present invention, an electric circuit is preferably formed on the surface or inside of the glass-ceramic. It is permissible that an electric circuit may be formed on the surface of the aluminum nitride sintered body.
As an electric circuit, any of known products containing conductors, resistor materials, dielectric materials and the like can be used without any limitation. Methods of its formation again may be those known ones.
For forming an electric circuit, low-resistance and low-melting point metals, e.g., Au, Ag and Cu; high-melting point metals, e.g., tungsten and molybdenum; and other various metal materials such as platinum group, nickel, chromium, cobalt, titanium, zirconium, tantalum, niobium and their alloys; nitrides, carbides and silicates of those metals are used. Of those various materials, low-resistance materials such as Au, Ag, Cu and the like are most widely used. In order to avoide fusion of these metallic components, it is desirable that they are not heated at temperatures higher than 1100xc2x0 C., in particular, higher than 1050xc2x0 C., at the time of the circuit formation.
As methods for forming electric circuit, known methods such as screen printing using a metallic paste, electrolytic or non-electrolytic plating or thin film forming by means of sputtering or vapor deposition can be used.
When an electric circuit is to be formed on surface of the aluminum nitride sintered body, a circuit or circuits can be formed on one surface or plural surfaces (e.g., two surfaces facing with each other) of aluminum nitride sintered body, in advance of producing a jointed body of glass-ceramic with aluminum nitride sintered body. In that case, the glass-ceramic may be joined in any manner as will or will not cover the electric circuit on the aluminum nitride sintered body surface, or will cover only a part of the electric circuit. Furthermore, the glass-ceramic can be joined in such a manner as will cover one surface only of the aluminum nitride sintered body, or cover plural surfaces of the body. In the latter case, it is one of favorable embodiments to form electric circuits on plural surfaces of an aluminum nitride sintered body, and to cover some of the circuits with glass-ceramic, leaving the remaining circuits uncovered. It is also possible to so effect the joining as will cover only a part, not the entiety, of any one surface.
Glass-ceramic may also be joined not in contact with an electric circuit in the occasion of soldering surfaces of the material constituting an electric circuit for preventing corrosion of said material, with the view to prevent fused solder from flowing into the parts not requiring soldering.
When an electric circuit is to be formed on the glass-ceramic surface, it can be performed in the manner similar to above-described methods for forming an electric circuit or circuits on aluminum nitride sintered body surface. As a method for forming an electric circuit inside of the glass-ceramic, the one comprising first forming a layer of a substance capable of forming an electric circuit under heating on the surface of the starting glass, heating the laminate to convert it to a glass-ceramic with a surface on which an electric circuit layer is formed, then further forming a glass layer on said electric circuit layer to cover the latter, heating the whole once again to integrate the glass, and repeating the above-described procedures. It is preferred for thus prepared glass laminate to be so well integrated that the jointed portions are fused and visually indistinguishable. In certain cases, however, the integration is incomplete, allowing confirmation of each glass layer and junctions therebetween. This invention also encompasses such an embodiment.
In the occasion of laminating glass at multiple stages, glasses of different compositions may be used for individual layers. For example, for the layer contacting with aluminum nitride sintered body, a glass exhibiting high jointability and having a low softening point can be used, while for the layer contacting the atmospheric air, a glass of high chemical resistance can be used. Furthermore, within an extent not impairing the effects of the present invention, a glass not containing the crystal having the strongest line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line can be concurrently used. An example of such a case is illustrated as over-glass 4 in later discussed FIG. 20.
Among these methods for forming electric circuit, furthermore, a preferred embodiment comprises applying a known conductive metallic paste onto an aluminum nitride sintered body surface and/or onto a glass layer formed of a glass paste which contains the amorphous starting glass, by such means as screen printing, and heating the laminate at 800-970xc2x0 C. to simultaneously effect conversion of the amorphous glass paste to glass-ceramic, joining the glass-ceramic with the aluminum nitride sintered body, and conversion of the conductive paste to an electric circuit.
In the above-described case of adopting multi-layered electric circuits and glass-ceramic construction, the glass-ceramic functions as an insulation layer between every two electric circuits. Thickness of the glass-ceramic layer serving as an insulation layer is preferably 1-300 xcexcm, in particular, 3-100 xcexcm, inter alia 5-70 xcexcm, per layer, for assuring good electrical insulation and facilitating formation of via hole(s) electrically connecting electric circuit layers and formation of uniform thickness.
The present invention also provides a method as above-described, which forms glass.ceramic, aluminum nitride sintered body and electric circuit(s) by heating.
In this case, the starting amorphous glass to be converted to glass-ceramic does not necessarily contain Zn component, it being sufficient for obtaining the effects of the invention that it is capable of separating under heating a crystal having the strongest line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 l and is substantially amorphous, containing in total not more than 10 wt % of Ti component and Zr component in terms of respective oxides, and not more than 5 wt % of Pb component in terms of its oxide. Needless to say, the amorphous glass preferably contains 0.5-30 wt % of Zn component, so that the heating temperature for the jointed body production can be lowered.
In a preferred embodiment of the jointed body according to the invention, which is formed of glass-ceramic and aluminum nitride sintered body, the glass-ceramic contains 30-80 wt % of crystalline portion, 60-90 wt % of said crystalline portion being the crystals having the strongest line in a range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line, and 10-40 wt % of said crystalline portion being the crystals having the strongest line in the range of 2xcex8=36.6xc2x0-37.0xc2x0 in powder X-ray diffraction using CuKxcex1 line; said glass-ceramic has a composition composed of CaO: 8-25 wt %, Al2O3: 15-35 wt %, SiO2: 33-55 wt %, ZnO: 0.5-25 wt % and B2O3: 0.05-18 wt %, being a CaOxe2x80x94Al2O3xe2x80x94SiO2xe2x80x94B2O3xe2x80x94ZnO glass-ceramic substantially free of other component; an average jointing strength at the glass-ceramic-aluminum nitride sintered body interface, as measured by 90xc2x0 perpendicular tensile test, is at least 35 MPa; and electric circuits are formed in multi-layers on the surface and/or inside of the glass-ceramic.
A particularly preferred embodiment is a jointed body in which the crystalline portion of the glass-ceramic is 40-70 wt % wherein anorthite crystals occupying 70-90 wt % and gahnite crystals occupying 10-30 wt %; the glass-ceramic has an electric resistance of at least 1xc3x971014 xcexa9.cm and dielectric constant not higher than 7 at 1 GHz; the average jointing strength of the glass-ceramic and aluminum nitride sintered body at their interface, as measured by 90xc2x0 perpendicular tensile test is at least 50 MPa; and electric circuits are formed in multi-layers on the surface and/or inside of the glass-ceramic.
FIG. 20 shows an example of a circuit substrate. Referring to FIG. 20, a three layered glass-ceramic 1 is formed on an aluminum nitride sintered substrate 2. At inside and on surface of the glass-ceramic and on surfaces of the aluminum nitride sintered substrate, wiring conductors 3 are formed. For these wiring conductors, low resistance material made chiefly of Cu, Ag and Au is useful. Besides, cover glass 4 for protecting the wiring conductors and via 6 for conductivity are formed. This circuit substrate carries semiconductor chip 5 and chip parts 7 such as resistor and capacitor.
Jointed bodies of the present invention are not limited to the one illustrated on FIG. 20. For example, the glass layer may be formed of one, two, or four or more layers. Also semiconductor chip mounting is preferably conducted directly on aluminum nitride sintered sabstrate without a glass layer intervening, as shown on FIG. 20 for better heat spreading, while it is possible to mount it through a thin glass layer where circuit design requires it. The thickness of the glass layer in such an occasion is preferably not more than 100 xcexcm. The semiconductor chip can be mounted not on the glass layer side as shown in FIG. 20, but on the opposite side.
In the present invention, the glass-ceramic containing a crystal having the strongest line in the range of 2xcex8=27.6xc2x0-28.2xc2x0 in powder X-ray diffraction using CuKxcex1 line exhibits, besides chemical resistance, excellent oxidation resistance, plasma etching resistance and the like, and upon being joined with aluminum nitride sintered body, prevents the body from direct exposure to severe environments and contributes to improvements in chemical resistance (in particular, alkali resistance or corrosion resistance by molten salts), oxidation resistance (in particular, corrosion resistance in an oxidizing atmosphere at high temperatures not lower than 600xc2x0 C.) and plasma etching resistance (in particular, resistance of semiconductor production apparatus to corrosion by plasma gas containing corrosive elements such as chlorine and fluorine) of the aluminum nitride sintered body. Jointed bodies of glass-ceramic and aluminum nitride sintered body according to the present invention, therefore, are applicable not only to above circuit substrate but also to mechanical parts or high temperature structural material for heater or the like or tooling for semiconductor-producing apparatus.